Hemodynamics derived from transesophageal echocardiography

INTRODUCTION — Transesophageal echocardiography (TEE) can rapidly provide a comprehensive array of hemodynamic information. This is of particular importance in the critical care setting, where patients are often obtunded and mechanically ventilated; TEE may aid in differentiating among causes of hypotension, etiologies of dyspnea, and causes of chest pain. To reliably and safely apply TEE to critical care, skill, experience, and multidisciplinary approach are essential. A working relationship among intensivists, anesthesiologists, and cardiologists is an integral part of every successful critical care TEE program.

This topic will review the hemodynamic information which can be obtained during TEE. The general indications for TEE, along with its use in specific clinical situations, are discussed separately. (See "Transesophageal echocardiography: Indications, complications, and normal views" and "Transesophageal echocardiography in the evaluation of the left ventricle" and "Transesophageal echocardiography in the evaluation of mitral valve disease" and "Transesophageal echocardiography in the evaluation of aortic valve disease".)

HEMODYNAMIC VARIABLES — Hemodynamic variables that are derived from TEE are comprehensive enough to provide accurate information [1]; variables that can be estimated include:

●Cardiac output

●Left ventricular (LV) filling pressure

●Temporal distribution of LV filling

●Chamber preload

●Atrial interaction

●Pulmonary pressure

Cardiac output — Measurements of cardiac output based on TEE may be variable and should be considered in the context of other available information.

Doppler-derived pulmonary artery, aortic, or mitral flow signals can be used to calculate stroke volume, based on the principle that the velocity time integral (VTI) of blood flow multiplied by the cross-sectional area at any of these conduits (cm²) estimates the cardiac stroke volume. The product of stroke volume and heart rate is cardiac output.

The VTI is the actual area under the Doppler signal obtained or the velocity-time curve; because the Doppler signal is velocity (distance/time), the area under the velocity-time curve is distance/time x time or distance. Thus the velocity time integral is referred to as the stroke distance which is the distance traveled by the sampled volume within each heartbeat. Stroke distance is normally 13 to 15 cm for the pulmonary artery and 18 to 20 cm for the aorta [2,3]. Use of this parameter may eliminate the error introduced by estimating the cross-sectional area of the vessel as the VTI holds constant over a broad range of body surface areas [4]. For measurements of stroke distance and the derived cardiac output, it is important that the Doppler beam interrogation angle is within 20º of the direction of flow. (See "Principles of Doppler echocardiography", section on 'Basic principles'.)

The use of TEE to determine cardiac output by each of these methods has been investigated. The main pulmonary artery is the most convenient site from which to measure the VTI. In the basal short axis view, pulmonary artery diameter can be measured in most patients and flow signals can be obtained using either pulsed or continuous wave Doppler. This Doppler method was established in the operating room by comparing it with thermodilution cardiac output [5]. Although cardiac output by thermodilution is not an ideal reference standard, the pulsed-wave Doppler method was able to follow directional changes in that parameter.

Aortic blood flow cannot usually be accurately measured from the esophageal views because of poor ultrasound beam alignment for Doppler signals. However, using the "deep transgastric view" aligns the LV outflow tract with the transducer to obtain accurate flow signals and permits accurate determination of the cardiac output [6,7]. In one intraoperative TEE study, cardiac output by Doppler was compared with thermodilution cardiac output and was shown to have close agreement [8]. In addition, the Doppler method was accurate in detecting more than 10 percent of change in cardiac output. However, another more recent study showed relatively poor agreement between TEE estimates of cardiac output and thermodilution [9].

Measurements of cardiac output based on mitral flow correlate well with thermodilution cardiac output when biplane TEE measurements of the mitral annulus in the two- and four-chamber views are used to calculate the cross-sectional area as an ellipse [10,11].

Left ventricular and atrial filling pressures — Pulmonary venous flow patterns, mitral inflow signal configuration, and continuous-wave Doppler of mitral regurgitation contain information from which filling pressures can be estimated (figure 1).

Doppler mitral inflow velocity — Doppler demonstration of the velocity profile of LV transmitral inflow (figure 1) is the most informative TEE-based method of assessing LV filling pressures (waveform 1A-B). Important diagnostic parameters derived from the mitral inflow signals include the ratio of peak early filling velocity to late diastolic atrial filling velocity (E/A ratio), the deceleration time of early filling curve (DT), and the isovolumic relaxation time (IVRT). Studies have demonstrated that the relationship between peak velocity and deceleration in early diastole with late diastolic velocities during atrial contraction is altered in diastolic dysfunction [12-14].

Although there is considerable variability, predominant patterns of flow have been recognized (image 1A-D) that reflect the major categories of diastolic dysfunction, reflecting impaired LV relaxation and decreased LV compliance [15]. (See "Echocardiographic evaluation of left ventricular diastolic function in adults", section on 'Mitral inflow velocities and isovolumic relaxation time'.)

●Impairment of LV relaxation is characterized by a reduction in mitral flow velocity in early diastole, manifested by an abnormally low E wave, and increased late diastolic filling, manifested by an increased A wave; the E/A ratio is decreased and is <1 compared with normal where the E/A ratio is ≥1. Additionally, there is prolongation of the deceleration time and the isovolumic relaxation times. This form of diastolic dysfunction occurs in ischemic heart disease, hypertension, and as a result of normal aging. This pattern is usually associated with normal filling pressures.

●Diminished LV compliance is characterized by a "restrictive flow" pattern with an increased E/A ratio and a shortened isovolumic relaxation time and deceleration time. This pattern occurs in patients with restrictive cardiomyopathies, such as amyloid heart disease, and in those with elevated filling pressures associated with a variety of myopathic conditions.

However, the diastolic filling pattern does not always reflect these two circumscribed categories because it is influenced by a variety of other factors, including loading conditions, heart rate, pericardial restraint, left atrial pressure and compliance, right and LV interaction, coronary arterial compliance, the intrinsic properties of left atrial and LV muscle, the presence or absence of mitral regurgitation, and patient age. For example, changes in loading conditions during cardiac surgery have been shown to influence filling patterns [16]. With an increase in intravascular volume, a rise in early diastolic filling velocity is seen, most likely due to an increased left atrial to LV pressure gradient [17]. Similarly, an increase in left atrial pressure due to ischemia may be associated with an increase in early diastolic filling velocities, ie, pseudonormalization [18]. Pseudonormalization is an intermediate pattern, which reflects an increase in filling pressures. The E velocity increases, and the deceleration time decreases.

In contrast, hypovolemia or preload reduction (eg, nitrates) may cause a decrease in early filling velocity mimicking impaired relaxation.

Pulmonary venous flow — The interpretation of the mitral inflow pattern is complimented by examination of the pulmonary venous Doppler (figure 1); TEE has contributed substantially to our understanding of this hemodynamic parameter. (See "Echocardiographic evaluation of left ventricular diastolic function in adults", section on 'Pulmonary venous flow'.)

TEE from the base of the heart demonstrates the entrance of the four pulmonary veins into the left atrium. The left upper pulmonary vein flow is close to and parallel to the direction of the interrogating beam and highly resolved color flow pulsed-wave Doppler images are readily obtained, helping to guide positioning of the sample volume into the proximal 1 cm of the vein.

Flow in the pulmonary veins is triphasic [17-19]. The systolic phase is normally predominant in most adults, accounting for more than 55 percent of total flow integral. The second phase occurs in early diastole and is approximately 40 percent of the total. Both of these phases move in an antegrade, central direction and occur during atrial systolic filling and atrial diastolic emptying. The third phase is the late diastolic reversal that results from atrial contraction. In contrast to the first two phases, the late diastolic phase is usually brief and lower in velocity (figure 2 and image 2 and image 3 and figure 3).

Variables frequently measured from pulmonary venous flow velocity tracings with demonstrated utility include:

●Peak systolic and peak early diastolic flow velocities

●Peak velocities of flow reversal at atrial contraction

●Velocity-time integrals of the systolic, early diastolic, and atrial contraction phases

The systolic velocity-time integral (S) is measured from the onset of forward flow following the peak R wave on the electrocardiogram to the point at which it reaches zero flow velocity. The early diastolic velocity-time integral (D) is measured from the onset of the second wave to its crossover with the zero-line. The systolic fraction is equal to S/(S + D). The velocity-time integral of the late diastolic A wave (retrograde flow during atrial contraction) is measured from its onset to the end of negative flow.

In conditions that alter LV filling patterns, especially when filling pressures are elevated, systolic filling is reduced relative to diastolic filling. This finding is most reliable in patients with reduced LV ejection fraction [20]. Concurrently, retrograde atrial flow may increase in both velocity and duration, and it exceeds forward atrial flow across the mitral valve in duration [21]. In severe mitral regurgitation, systolic flow reversal has been described as one of the signs of severity [22].

Mitral inflow and pulmonary venous flow patterns are complementary [16,20]. For example, restrictive mitral inflow is characterized by a short isovolumic relaxation time, a normal or slightly elevated peak velocity, a short deceleration time (<150 msec), and a low A wave velocity. A restrictive pattern on mitral inflow and decreased systolic fraction on pulmonary venous inflow are complementary, permitting confident recognition of elevated filling pressure. A prolonged retrograde pulmonary venous A wave and a shortened A wave on mitral inflow also connote elevated pressure. (See "Echocardiographic evaluation of left ventricular diastolic function in adults".)

The use of tissue Doppler parameters to assess left sided filling pressures during TEE has been disappointing. E/e', where E is the mitral inflow velocity and e' is the early recoil of the mitral annulus in diastole, has not been shown to correlate with left atrial pressure intraoperatively in mechanically ventilated patients and should probably not be used [23].

Mitral regurgitation continuous-wave Doppler echocardiography — Features of the mitral regurgitant continuous-wave Doppler flow signal are the density of the signal, which is roughly proportional to severity of regurgitation; the v-wave cutoff sign, which results from a rapid decrease in ventriculoatrial gradient as the regurgitant blood abruptly raises pressure in the left atrium; and the early systolic acceleration of the jet flow envelope, which is an expression of dP/dt (the change in pressure over time) The normal dP/dt is >1000 mmHg/second. (See "Echocardiographic evaluation of the mitral valve".)

Left ventricular and atrial chamber sizes (preload) — Volume changes (ie, preload) affect the size and shape of the left heart chambers.

Hypovolemia — In acute hypovolemia, most normal hearts become hyperdynamic and develop very small end-systolic volumes. Rarely, in preload sensitive hearts, contractility may decrease. The left atrium in hypovolemia may decrease in size. Similarly, the right atrium, venae cavae, and hepatic veins become small, and respiratory collapse might be appreciated.

Hypervolemia — In hypervolemia, as seen in congestive heart failure, especially if chronic, the left ventricle assumes a spherical state as it remodels and dilates. This appearance is strikingly different from the ellipsoid shape of the healthy heart. While one cannot determine the filling pressure from inspecting the ventricles and atria, observations of these features can reinforce the inferences drawn from Doppler data.

Intra-atrial septal motion — The behavior of the interatrial septum may provide information as to the relative right and left sided filling pressure [24]. Observations made on ventilated patients showed that in the euvolemic or hypovolemic state, the interatrial septum, normally curved to the right, will reverse curvature at end expiration at both end-systole and end-diastole. The cause of the rapid reversal in curvature has been documented with flow-directed catheters; it arises from a transient reversal in the pressure differential in such a way that right atrial pressure transiently exceeds left atrial pressure during the expiratory phase of the ventilator cycle. This occurs only when pressures (pulmonary capillary wedge and central venous pressure) are low and nearly equal.

If either atrium carries higher pressure, the atrial septum will remain bowed toward the lower pressure chamber. In mitral regurgitation, for example, the atrial septum bows from left to right, and this curvature is little affected by the respiratory cycle. In tricuspid regurgitation or pulmonary hypertension associated with increased right sided filling pressures, the curvature goes from right to left. It is worth emphasizing how useful this simple observation can be in integrating multiple variables and emerging with a clear picture of a patient's hemodynamics.

Pulmonary artery pressure — As in TTE, the right ventricular systolic pressure can be estimated from the peak velocity of the tricuspid regurgitant jet applying the modified Bernoulli equation. In the absence of RV outflow tract obstruction, this value, when added to a measured central venous pressure, is equivalent to the pulmonary artery systolic pressure. It is critical that the TR jet is interrogated at an optimal angle and multiple views should be used to find the one that is most parallel to flow, especially in the presence of an eccentric jet. The size and respiratory dynamics of the inferior vena cava cannot be used in mechanically ventilated patients so central venous pressure must be measured directly [25].

SUMMARY

●Hemodynamic variables that can be estimated during transesophageal echocardiography (TEE) include cardiac output, left ventricular (LV) filling pressure, temporal distribution of LV filling, chamber preload, atrial interaction, and pulmonary arterial pressures. (See 'Hemodynamic variables' above.)

●Doppler-derived pulmonary artery, aortic, or mitral flow signals can be used to calculate stroke volume, based on the principle that the velocity time integral (VTI) of blood flow multiplied by the cross-sectional area at any of these conduits (cm²) estimates the cardiac stroke volume. However, there is considerable variability in these measurements, and they must be considered in the context of the patient’s overall clinical status. (See 'Cardiac output' above.)

●Doppler demonstration of the velocity profile of LV transmitral inflow is the most informative TEE-based method of assessing LV filling pressures. However, the transmitral diastolic filling pattern does not always reflect underlying pathology because it is influenced by a variety of other factors (eg, loading conditions, heart rate, pericardial restraint, left atrial pressure and compliance, patient age, mitral regurgitation, and stenosis). (See 'Doppler mitral inflow velocity' above.)

●Doppler investigation of the pulmonary venous flow patterns is another method of evaluating left-sided filling pressures and loading conditions, providing complementary information to the transmitral inflow profile especially in patients with reduced LV ejection fraction. (See 'Pulmonary venous flow' above.)

●Chamber sizes, particularly if abnormal, provide information about the preload conditions in the heart as well as potentially providing information on any underlying pathology. (See 'Left ventricular and atrial chamber sizes (preload)' above.)

●The behavior of the interatrial septum is a particularly important clue as to the intracardiac filling pressures. If either atrium carries higher pressure, the atrial septum will remain bowed toward the lower pressure chamber. (See 'Intra-atrial septal motion' above.)